The present invention relates to a plate have orifices or nozzles therethrough used in particle classification impactors that classify particles according to size, and in particular relates to large plates used as part of the impactor or as a final filter where there are a large number of small nozzles or orifices. When the nozzles are very small the plate must be of a thin material. The nozzle plate has a selected number of dimples protruding from the surface that faces a surface to insure that a space is maintained between the plate and the surface through which exhaust fluid will flow after passing through the nozzles or orifices.
Cascade impactors that have relatively large diameter plates with small openings, are known in the field. Cascade impactors are illustrated in the co-pending U.S. Patent Application identified above.
In the final stages of a cascade impactor, particularly those used for analyzing dry powder inhalers, the particles that are being classified are relatively small, which requires small nozzles or orifices. Also a final filter can be provided which uses a thin plate with small openings. In order to have a nozzle plate that is effective with very small nozzles or orifices, the plate has to be thin, typically a few thousands of an inch. For large diameter plates that means the binding strength is relatively low, so when the nozzle plate is supported at its edges above a closely spaced impactor surface, the nozzle plate may bow under differential pressure caused by flow through the impactor. Such plates may. have as many as 2000 nozzles, and more may be needed in the future. The thin plate may bow enough to contact the impactor surface in the center portions of the nozzle plate, thereby reducing the number of effective orifices or nozzle openings and causing problems with appropriate flow and particle classification.
The present invention relates to impactor and filter plate contructions for cascade impactors where the plate is thin, and is supported at its peripheral edges above an impactor plate or a collection cup. Typically, a plurality of very small nozzle openings or orifices are formed in the nozzle or filter plate.
The plate of the present invention, which will be called a nozzle or orifice plate, has one or more small protrusions extending from the surface of the nozzle plate that faces a surface such as in a particle collector, cup or on an impactor plate. If the nozzle plate bows, the ends of the protrusions will contact the cup or plate surface and provide support for the nozzle plate at locations that are closely spaced enough to support the majority of the nozzle plate spaced from the cup or impactor plate surface. The protrusions insure that there is a passageway for a layer of air to flow between the nozzle plate and the impactor plate or cup.
In the preferred form, the protrusions are formed by dimpling the thin nozzle plate to form small projections from the surface of the plate that overlies the impactor plate or collection cup.
The use of the dimples is particularly helpful where there is a high differential pressure across the nozzle plate. Again, this is usually in connection with the later stages of a cascade impactor, or as part of a final filter.
Greater differential pressures are encountered in newer impactors, because of the requirements for relatively high air flows and small nozzles.